The present invention relates in general to internal combustion engines for automotive vehicles and, particularly, to a method of controlling the fuel-to-air ratio of the combustible mixture in an automotive internal combustion engine for exhaust emission control purposes. The present invention is also concerned with an automotive internal combustion engine in which such a method is put into practice.
The chemical composition of exhaust gases from the power cylinders of an internal combustion engine is a faithful representation of the air-to-fuel ratio of the combustible mixture produced in the mixture supply system such as a carburetor of the engine. It has therefore been proposed and put into practice in some modernized versions of automotive internal combustion engines to have the fuel-to-air ratio of the combustible mixture controlled upon detection of the chemical composition of the exhaust gases from the power cylinders of an engine in an attempt to control exhaust emissions of the engine. A mixture ratio control system of this nature is useful especially when incorporated into an internal combustion engine of the type which is equipped with a thermal reactor or a catalytic converter in the exhaust system of the engine for re-combusting the combustible residues in the exhaust gases or converting specific types of toxic compounds such as hydrocarbons, carbon monoxide and nitrogen oxides in the exhaust gases into harmless compounds. As is well known in the art, a thermal reactor or a catalytic converter thus used to clean the exhaust gases of an internal combustion engine is adapted to exhibit its maximum performance efficiency when the exhaust gases passed therethrough have a specific chemical composition resulting from a predetermined fuel-to-air ratio such as typically a stoichiometric mixture ratio. For the purpose of achieving a maximum exhaust-gas cleaning efficiency in an internal combustion engine equipped with a thermal reactor or a catalytic converter in the exhaust system thereof, it is therefore important to have the air-to-fuel ratio of combustible mixture regulated to remain at a predetermined target value or within a certain range containing a predetermined target value that will result in exhaust gases having such a specific chemical composition throughout the various modes of operation or under prescribed modes of operation of the engine. The chemical composition of the exhaust gases to be detected is usually represented by the concentration of oxygen in the exhaust gases although the concentrations of other types of chemical components such as hydrocarbons, carbon monoxide and carbon dioxide are also closely related to the composition of the exhaust gases from an internal combustion engine.
A known mixture ratio control system adapted to regulate the fuel-to-air ratio toward a predetermined value in an automotive internal combustion engine using a carburetor as the mixture supply system has provided in the carburetor an extra air inlet passageway leading to a fuel delivery circuit of the carburetor and a solenoid-operated air-flow control valve which is adapted to close and open the air inlet passageway on the basis of digital signals representative of the chemical composition of the exhaust gases emitted from the power cylinders of the engine. The fuel to be delivered from the fuel delivery circuit and accordingly the fuel-to-air ratio of the combustible mixture to be finally produced in the carburetor are thus regulated by varying the flow rate of air to be injected into the fuel in the fuel delivery circuit through the additional air inlet passageway so that the suction developed in the fuel delivery circuit, viz., the motivating force to draw fuel from the fuel source is varied with the flow rate of air thus entering the fuel delivery circuit. When the fuel-to-air ratio of the combustible mixture produced in the carburetor is found to be lower than a predetermined target value or, in other words, the air-fuel mixture is found to be leaner than a mixture having a desired fuel-to-air ratio, the air-flow control valve is operated to close the air inlet passageway at an increased frequency and/or prolonged durations so as to reduce the flow rate of air through the air inlet passageway and enrich the air-fuel mixture to be produced in the carburetor until the fuel-to-air ratio of the mixture as monitored from the exhaust gases resulting from the mixture is found to have reached the target value. If, conversely, the fuel-to-air ratio of the combustible mixture produced in the carburetor is found to be higher than the predetermined target value, viz., the mixture is found to be richer than a mixture having the desired fuel-to-air ratio, then the air-flow control valve is operated to open up the air inlet passageway at a reduced frequency and/or for shortened durations so that air is injected into the fuel in the fuel delivery circuit at a reduced rate to produce enrichment of the air-fuel mixture until the fuel-to-air ratio of the mixture as monitored from the exhaust gases is found to have reached the target value.
In order to exploit the advantages of a mixture ratio control system of this nature, it is of critical importance that the delivery rate of fuel from the fuel delivery circuit be increased or decreased as fast as possible in response to a decrease or an increase, respectively, in the flow rate of air into the fuel delivery circuit. In a conventional mixture ratio control system of the described character, however, the motivating force to draw fuel from the fuel source is solely the suction developed in the fuel delivery circuit and, for this reason, the rate at which fuel is to be discharged from the fuel delivery circuit is varied only in response to changes in the degree of the suction in the circuit. Thus, the flow rate of fuel through the fuel delivery circuit cannot be increased or decreased instantaneously and distinctly in response to the flow of air into the fuel delivery circuit. This causes retardation in the changes in the fuel delivery rate in the carburetor and accordingly in the changes in the fuel-to-air ratio of the combustible mixture produced in the carburetor. The retardation in the changes in the fuel-to-air ratio results in fluctuations of the fuel-to-air ratio across the target value and critically impairs the potential function of the mixture ratio control system. The present invention contemplates elimination of such a drawback which is inherent in a prior-art mixture ratio control system of the described type.